Update the default archetype to specify all relevant frontmatter fields.
Updated the bigimg and comments features to handle "unset" or "null"
values in the frontmatter and default appropriately. Also updated the
bigimg sample to explain both the default and "flow" YAML syntax.

Add support for bigimg's on the front page, multiple images that rotate
on both the front page and regular pages and posts, also added test
images and a sample to demonstrate the functionality and how to specify
bigimg's.